The N1-L3 version was developed to compete with the United States Apollo-Saturn V to land a man on the Moon, using the same lunar orbit rendezvous method. The basic N1 launch vehicle had three stages, which was to carry the L3 lunar payload into low Earth orbit with two cosmonauts. The L3 contained an Earth departure stage; another stage used for mid-course corrections, lunar orbit insertion, and powered descent initiation; a single-pilot LK Lander spacecraft; and a two-pilot Soyuz 7K-LOK lunar orbital spacecraft for return to Earth. The Apollo spacecraft was able to carry three astronauts (landing two on the Moon), and relied on the Saturn V's third stage for Earth departure.

N1-L3 was underfunded and rushed, starting development in October 1965, almost four years after the Saturn V. The project was badly derailed by the death of its chief designer Sergei Korolev in 1966. Each of the four attempts to launch an N1 failed; during the second launch attempt the N1 rocket crashed back onto its launch pad shortly after liftoff and exploded, resulting in one of the largest artificial non-nuclear explosions in human history. The N1 program was suspended in 1974, and in 1976 was officially canceled. Along with the rest of the Soviet manned lunar programs, the N1 was kept secret almost until the collapse of the Soviet Union in December 1991; information about the N1 was first published in 1989.

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Development began under the direction of Sergei Korolev at his OKB-1 Design Bureau. The original design proposed a 50-metric-ton (110,000 lb) payload[7] intended as a launcher for military space stations and a crewed flyby of Venus and Mars in the TMK (Russian acronym for Heavy Interplanetary Spacecraft) using a nuclear engine upper stage. The N1 was the largest of three proposed designs; the N2 was somewhat smaller and intended to compete with Vladimir Chelomei's proposed UR-200, and the much smaller N3, which would replace Korolev's "workhorse" R-7 rocket. At this point the N-series was strictly a "paper project".

In December 1959, a meeting was called with all of the chief designers, who presented their latest designs to the military. Korolev presented the N-series along with a much more modest series of upgrades to the R-7. Vladimir Chelomei, Korolev's rival, presented his "Universal Rocket" series, which used a common lower stage in various clustered configurations to meet a wide variety of payload requirements. Mikhail Yangel, perhaps the most successful of the three but with little political power, presented the small R-26 intended to replace the R-16, the much larger R-36 ICBM, as well as the SK-100, a space launcher based on a huge cluster of R-16's. In the end the military planners selected Chelomei's UR-100 as the new "light" ICBM, and Yangel's R-36 for the "heavy" role. They saw no need for any of the larger dedicated launchers, but also gave Korolev funding to develop the Molniya (8K78) adaptation of the R-7.

In March 1961, during a meeting at Baikonur, designers discussed the N1 design, along with a competing Glushko design, the R-20. In June, Korolev was given a small amount of funding for N1 development between 1961 and 1963. In May 1961 a government report, On Reconsideration of the Plans for Space Vehicles in the Direction of Defense Purposes, set the first test launch of the N1 rocket for 1965.

When the US announced in May 1961 the goal of landing a man on the Moon, Korolev proposed a lunar mission based on a new spacecraft, eventually known as Soyuz, that was designed for Earth orbit rendezvous. Several launches would be used to build up a complete moon package, one for the Soyuz, another for the lunar lander, and additional launches with cislunar engines and fuel. This approach makes the least demands on the launch vehicle, as the payload mass is reduced for any one launch. This is at the expense of requiring a rapid launch rate to ensure that the modules are built up before running out of consumables while waiting on-orbit. Even using this profile the lunar boosters and fuel were too large for any existing Soviet launcher, and Korolev proposed the N1 be used for this role.

Korolev felt that the toxic nature of the fuels and their exhaust presented a safety risk for crewed space flight, and that kerosene/LOX was a better solution. Glushko pointed out that the US Titan rockets used to launch Gemini spacecraft also used UDMH/N2O4 propellants. The Americans also had a 5-year head start with F-1 engine development, and were still facing combustion stability problems. Glushko held it was unrealistic and unfair to expect him to stake his reputation on miraculously delivering a similar engine virtually overnight with practically no money, primitive computer technology and an inferior kerosene fuel prone to coking (leaving contaminating deposits of unburned carbon) at high temperatures, as opposed to the rocket-grade kerosene used in the Saturn V.

There were strong personal resentments between the two, Korolev holding Glushko responsible for his near-death at Kolyma Gulag and the failure of his first marriage as a result, and Glushko considering Korolev to be irresponsibly cavalier and autocratic in his attitudes towards things outside his competence. Glushko refused outright to work on LOX/kerosene engines, and with Korolev in general. He instead teamed up with other rocket designers to build the very successful Proton rocket, Zenit rocket and Energia rocket.

Later, Glushko did build a four-chamber LOX/Kerosene engine even more powerful and advanced than the F-1, known as the RD-170. Its development took over ten years, despite it being 20 years after the American F-1, due to the relative backwardness of the USSR's industrial base as foreseen by Glushko. This probably vindicated his decision not to support the development of such an engine for the N1 rocket.

The difference of opinions led to a falling out between Korolev and Glushko. In 1962, a committee that was appointed to break the logjam agreed with Korolev. Since Glushko refused to work on such a design, Korolev eventually gave up and decided to enlist the help of Nikolai Kuznetsov, the OKB-276jet engine designer.

Kuznetsov, who had limited experience in rocket design, responded with a fairly small engine known as the NK-15, which would be delivered in several versions tuned to different altitudes. To achieve the required amount of thrust, it was proposed that a large number of NK-15s would be used in a clustered configuration around the outer rim of the lower-stage booster. The "inside" of the ring of engines would be open, with air piped into the hole via inlets near the top of the booster stage. The air would be mixed with the exhaust in order to provide thrust augmentation, as well as additional combustion with the deliberately fuel-rich exhaust. The ring-like arrangement of so many rocket engine nozzles on the N1's first stage could have been an attempt at creating a crude version of a toroidal aerospike engine system; more conventional aerospike engines were also studied.

Meanwhile, Chelomei's OKB-52 proposed an alternate mission with much lower risk. Instead of a crewed landing, Chelomei proposed a series of circumlunar missions which he felt would be able to beat the US. He also proposed a new booster for the mission, clustering three of his existing UR-200 designs (known as the SS-10 in the west) to produce a single larger booster, the UR-500. These plans were dropped when Glushko offered Chelomei the RD-270, which allowed the construction of a much simpler "monoblock" design, also known as the UR-500. He also proposed adapting an existing spacecraft design for the circumlunar mission, the single-cosmonaut LK-1. Chelomei felt that improvements in early UR-500/LK-1 missions would allow the spacecraft to be adapted for two cosmonauts.

The Soviet military, specifically the Strategic Missile Forces, was reluctant to support what was essentially a politically motivated project with little military utility, but both Korolev and Chelomei pushed for a lunar mission. For some time, between 1961 and 1964, Chelomei's less aggressive proposal was accepted, and development of his UR-500 and the LK-1 were given a high priority.

Since the US Project Gemini reversed the Soviet lead in human space exploration by 1966, Korolev was able to persuade Leonid Brezhnev to let him pursue his plans to make a lunar landing before the US.[citation needed] This required much larger boosters.

Korolev proposed a larger N1, combined with a new lunar package known as the L3. The L3 combined the lunar engines, an adapted Soyuz spacecraft (the LOK) and the new LK lunar lander in a single package. Chelomei responded with a clustered UR-500-derived vehicle, topped with the L1 spacecraft already under development, and a lander of their own design. Korolev's proposal was selected as the winner in August 1964, while Chelomei was told to continue with his circumlunar UR-500/L1 work.

When Khrushchev was overthrown later in 1964, infighting between the two teams started anew. In October 1965, the Soviet government ordered a compromise; the circumlunar mission would be launched on Chelomei's UR-500 using Korolev's Soyuz spacecraft in place of their own Zond design, aiming for a launch in 1967, the 50th anniversary of the Bolshevik Revolution. Korolev, meanwhile, would continue with his original N1-L3 proposal. Korolev had clearly won the argument, but work on the L1 continued anyway, as well as the Zond.

Korolev died in 1966 due to complications after minor surgery, and the work was taken over by his deputy, Vasily Mishin. Mishin did not have Korolev's political astuteness or power, a problem that led to the eventual downfall of the N1, and of the lunar mission as a whole.

The N1 was a very large rocket, standing 105 meters (344 ft) tall with its L3 payload. The N1-L3 consisted of five stages in total: the first three (N1) for insertion into a low Earth parking orbit, and another two (L3) for translunar injection and lunar orbit insertion. Fully loaded and fueled, the N1-L3 weighed 2,750 tonnes (6,060,000 lb). The lower three stages were shaped to produce a single frustum 17 meters (56 feet) wide at the base,[8] while the L3 section was mostly cylindrical, carried inside a shroud 3.5 meters (11 feet) (estimated) wide.[9] The conical shaping of the lower stages was due to the arrangement of the tanks within, a smaller spherical kerosene tank on top of the larger liquid oxygen tank below.

The first stage, Block A, was powered by 30 NK-15 engines arranged in two rings, the main ring of 24 at the outer edge of the booster and the core propulsion system consisting of the inner 6 engines at about half diameter.[10] The engines were the first ever staged combustion cycle engines. The control system was primarily based on differential throttling of the engines of the outer ring for pitch and yaw. The core propulsion system was not used for control.[11] The Block A also included four grid fins, which were later used on Soviet air-to-air missile designs. In total, the Block A produced 45,400 kN (10,200,000 lbf)[12][13][14] of thrust. This exceeded the 33,700 kN (7,600,000 lbf) thrust of the Saturn V.[15] The Saturn V used higher-specific impulseliquid hydrogen fuel in the second and third stages, which eliminated one of the stages needed to get to translunar injection, thus saving weight.

The second stage, Block B, was powered by 8 NK-15V engines arranged in a single ring. The only major difference between the NK-15 and -15V was the engine bell and various tunings for air-start and high-altitude performance. The upper stage, Block V (В/V being the third letter in the Russian alphabet), mounted four smaller NK-21 engines in a square.

During the N1's lifetime, a series of improved engines was introduced to replace those used in the original design. The first stage used an adaptation of the NK-15 known as the NK-33, the second stage a similar modification known as the NK-43, and finally the third stage used the NK-31. The resulting modified N1 was known as the N1F, but did not fly before the project's cancellation.

The KORD (Russian acronym for KOntrol Raketnykh Dvigateley—literally "Control (of) Rocket Engines"—Russian: Контроль ракетных двигателей)[16] was the automatic engine control system devised to throttle, shutdown and monitor the large cluster of 30 engines in Block A (the first stage). The KORD system controlled the differential thrusting of the outer ring of 24 engines for pitch and yaw attitude control by throttling them appropriately and it also shut down malfunctioning engines situated opposite each other. This was to negate the pitch or yaw moment diametrically opposing engines in the outer ring would generate, thus maintaining symmetrical thrust. Block A could perform nominally with two pairs of opposing engines shut down (26/30 engines), Block B with one pair of opposing engines shutdown (6/8 engines) and Block V with one engine shut down (3/4 engines). Unfortunately the KORD system was unable to react to rapidly occurring processes such as the exploding turbo-pump during the 5L launch.[17] Due to the deficiencies of the KORD system a new computer system was developed for the last launch, vehicle 7L, called the S-530. It was the first Soviet digital guidance and control system.[18] The telemetry system relayed data back at an estimated rate of 9.6 gigabytes per second on 320,000 channels on 14 frequencies. Commands could be sent to an ascending N1 at the same rate.[19]

A comparison of the U.S. Saturn V rocket (left) with the Soviet N1/L3. Note: human at bottom illustrates scale

At 105 meters (344 ft), the N1-L3 was slightly shorter and more slender overall, than the American Apollo-Saturn V (111 meters, 363 ft), but wider at the base (17 m/56 ft vs. 10 m/33 ft). The N1 produced more thrust in each of its three stages than the Saturn V. It also produced more total impulse in its first four stages than the Saturn V did in its three (see table below).

The N1 was intended to place the ≈95 t (209,000 lb) L3 payload into low Earth orbit,[20] whereas the Saturn V placed the roughly 45 t (100,000 lb) Apollo spacecraft, plus 74.4 t (164,100 lb) of fuel for translunar injection, into Earth parking orbit. L3 translunar injection of a 23.5 t (52,000 lb) payload was to be provided by the fourth stage. The N1-L3 would have been able to convert only 9.3% of its three-stage total impulse into Earth orbit payload momentum (compared to 12.14% for the Saturn V), and only 3.1% of its four-stage total impulse into translunar payload momentum, compared to 6.2% for the Saturn V.

The N1-L3 used only kerosene-based rocket fuel in all three of its stages, while the Saturn V used liquid hydrogen to fuel its second and third stages, which yielded an overall performance advantage due to the higher specific impulse. The N1 also wasted available propellant volume by using spherical propellant tanks under its conical-shaped external skin, while the Saturn V used most of its available cylindrical skin volume to house capsule-shaped hydrogen and oxygen tanks, with common bulkheads between the tanks in the second and third stages.

The Saturn V also had a superior reliability record: it never lost a payload in two development and eleven operational launches, while four N1 development launch attempts all resulted in failure, with two payload losses.

Complex plumbing was needed to feed fuel and oxidizer into the clustered arrangement of rocket engines. This proved to be extremely fragile, and was a major factor in the design's launch failures. Furthermore, the N1's Baikonur launch complex could not be reached by heavy barge. To allow transport by rail, all the stages had to be broken down and re-assembled. The engines for Block A were only test-fired individually and the entire cluster of 30 engines was never static test fired as a unit. Sergei Khrushchev stated that only two out of every batch of six engines were tested, and not the units actually intended for use in the booster. The reason for this was that the NK-15 engines had a number of valves that were activated by pyrotechnics rather than hydraulic or mechanical means, this being a weight-saving measure. Once shut, the valves could not be re-opened.[22] As a result, the complex and destructive vibrational modes (which ripped apart propellant lines and turbines) as well as exhaust plume and fluid dynamic problems (causing vehicle roll, vacuum cavitation, and other problems) in Block A were not discovered and worked out before flight.[23] Blocks B and V were static test fired as complete units.

Because of its technical difficulties and lack of funding for full-up testing the N1 never completed a test flight. All four uncrewed launches out of 12 planned tests ended in failure, each before first-stage separation. The longest flight lasted 107 seconds, just before first-stage separation. Two test launches occurred in 1969, one in 1971, and the final one in 1972.

Mishin continued with the N1F project after the cancellation of plans for a crewed Moon landing, in the hope that the booster would be used to build a moonbase. The program was terminated in 1974 when Mishin was replaced by Glushko. Two N1Fs were being readied for launch at the time, but these plans were canceled.

The program was followed by the "Vulkan" concept for a huge launch vehicle (with Syntin/LOX, later replaced by LH2/LOX as fuel on the 2nd and 3rd stages), and then in 1976, by the commencement of the Energia/Buran program.[24][25]

The two flight-ready N1Fs were scrapped and their remains could still be found around Baikonur years later used as shelters and storage sheds. The boosters were deliberately broken up in an effort to cover up the USSR's failed moon attempts, which was publicly stated to be a paper project in order to fool the US into thinking there was a race going on. This cover story lasted until glasnost, when the remaining hardware was seen publicly on display.

The advanced engines for the N1F escaped destruction. Although the rocket as a whole was unreliable, the NK-33 and NK-43 engines are considered rugged and reliable when used as a standalone unit. About 150 engines survived, and in the mid-1990s, Russia sold 36 engines to the US company Aerojet General for $1.1 million each and also license for the production of new engines.[29]

The US company Kistler Aerospace worked on incorporating these engines into a new rocket design, with which Kistler sought to eventually offer commercial launch services, before declaring bankruptcy. Aerojet also modified the NK-33 to incorporate thrust vector control capability for Orbital Science's Antares launch vehicle. Antares used two of the modified NK-33's, which Aerojet renamed the AJ-26, for first stage propulsion. The first four launches of the Antares were successful, but on the fifth launch the rocket exploded shortly after launch. Preliminary failure analysis by Orbital pointed to a possible turbopump failure in one NK-33/AJ-26. Given Aerojet's previous problems with the NK-33/AJ-26 engine during the modification and test program (two engine failures in static test firings, one of which caused major damage to the test stand) and the later in-flight failure, Orbital decided that the NK-33/AJ-26 was simply not reliable enough for future use.[30]

In Russia, N1 engines were not used again until 2004, when the remaining 70 or so engines were incorporated into a new rocket design, the Soyuz 3.[31][32] As of 2005[update], the project has been frozen due to the lack of funding. Instead, the NK-33 was incorporated into the first stage of a light variant of the Soyuz rocket, which was first launched on 28 December 2013.[33]

A few seconds into launch, a transient voltage caused the KORD to shut down Engine #12. After this happened, the KORD shut off Engine #24 to maintain symmetrical thrust. At T+6 seconds, pogo oscillation in the #2 engine tore several components off their mounts and started a propellant leak. At T+25 seconds, further vibrations ruptured a fuel line and caused RP-1 to spill into the aft section of the booster. When it came into contact with the leaking gas, a fire started. The fire then burned through wiring in the power supply, causing electrical arcing which was picked up by sensors and interpreted by the KORD as a pressurization problem in the turbopumps. The KORD responded by issuing a general command to shut down the entire first stage at T+68 seconds into launch. This signal was also transmitted up to the second and third stages, "locking" them and preventing a manual ground command from being sent to start their engines. Telemetry also showed that the power generators in the N-1 continued functioning until impact with the ground at T+183 seconds. Investigators discovered the remains of the rocket 32 miles (52 kilometers) from the launch pad. Vasily Mishin had initially blamed the generators for the failure, as he could not think of any other reason why all 30 engines would shut down at once, but this was quickly disproven by telemetry data and the recovery of the generators from the crash site. They had survived in good condition and were shipped back to the Istra plant, where they were refurbished and worked without any problems under bench testing. The investigative team did not speculate as to whether the burning first stage could have continued flying if the KORD system had not shut it down. The KORD was found to have a number of serious design flaws and poorly programmed logic. One unforeseen flaw was that its operating frequency, 1000Hz, happened to perfectly coincide with vibration generated by the propulsion system, and the shutdown of Engine #12 at liftoff was believed to have been caused by pyrotechnic devices opening a valve, which produced a high frequency oscillation that went into adjacent wiring and was assumed by the KORD to be an overspeed condition in the engine's turbopump. The wiring in Engine #12 was believed to be particularly vulnerable to this effect due to its length, however other engines had similar wiring and were unaffected. Also, the system ended up drawing 25V instead of its designed 15V due to the ruptured power lines. The control wiring was relocated and coated with asbestos for fireproofing and the operating frequency changed. [34][35] The launch escape system was activated and did its job properly, saving the mockup of the spacecraft. All subsequent flights had freon fire extinguishers installed next to every engine.[36][37] According to Sergei Afanasiev, the logic of the command to shut down the entire cluster of 30 engines in Block A was incorrect in that instance, as the subsequent investigation revealed.[38][39]

The second N-1 vehicle carried a modified L1 Zond spacecraft and live escape tower. Boris Chertok claimed that a mass model lunar module was also carried; however, most sources indicate that only the L1S-2 and boost stages were on board N-1 5L. Launch took place at 11:18 PM Moscow time. For a few moments, the rocket lifted into the night sky. As soon as it cleared the tower, there was a flash of light, and debris could be seen falling from the bottom of the first stage. All the engines instantly shut down except engine #18. This caused the N-1 to lean over at a 45-degree angle and drop back onto launch pad 110 East.[40] The nearly 2300 tons of propellant on board triggered a massive blast and shock wave that shattered windows across the launch complex and sent debris flying as far as 6 miles (10 kilometers) from the center of the explosion. Launch crews were permitted outside half an hour after the accident and encountered droplets of unburned RP-1 still raining down from the sky. The majority of the N-1's propellant load had not been consumed in the accident, and most of what had burned was in the first stage of the rocket. Moreover, the worst-case scenario, mixing of the RP-1 and LOX to form an explosive gel, had not occurred. The subsequent investigation revealed that up to 85% of the propellant on board the rocket did not detonate, reducing the force of the blast.[41] The launch escape system had activated at the moment of engine shutdown (T+15 seconds) and pulled the L1S-2 capsule to safety 1.2 miles (2 kilometers) away. Impact with the pad occurred at T+23 seconds. Launch Complex 110 East was thoroughly leveled by the blast, with the concrete pad caved in and one of the lighting towers knocked over and twisted around itself. Despite the devastation, most of the telemetry tapes were found intact in the debris field and examined.

Just before liftoff, the LOX turbopump in the #8 engine exploded (the pump was recovered from the debris and found to have signs of fire and melting), the shock wave severing surrounding propellant lines and starting a fire from leaking fuel. The fire damaged various components in the thrust section[42] leading to the engines gradually being shut down between T+10 and T+12 seconds. The KORD had shut off engines #7, #19, #20, and #21 after detecting abnormal pressure and pump speeds. Telemetry did not provide any explanation as to what shut off the other engines. Engine #18, which had caused the booster to lean over 45 degrees, continued operating until impact, something engineers were never able to satisfactorily explain. It could not be determined exactly why the #8 turbopump had exploded. Working theories were that either a piece of a pressure sensor had broken off and lodged in the pump, or that its impeller blades had rubbed against the metal casing, creating a friction spark that ignited the LOX. The #8 engine had operated erratically prior to shutdown and that a pressure sensor detected "incredible force" in the pump. Vasily Mishin believed that a pump rotor had disintegrated, but Alexander Kuznetsov argued that the NK-15 engines were entirely blameless and Mishin, who had defended the use of Kuznetsov's engines two years earlier, could not publicly come out and challenge him. Kuznetsov succeeded in getting the postflight investigative committee to rule the cause of the engine failure as "ingestion of foreign debris". Vladimir Barmin, chief director of launch facilities at Baikonur, also argued that the KORD should be locked for the first 15-20 seconds of flight to prevent a shutdown command from being issued until the booster had cleared the pad area.[43][44] The destroyed complex was photographed by American satellites, disclosing that the Soviet Union was building a Moon rocket.[37] After this flight, fuel filters were installed in later models.[37] It also took 18 months to rebuild the launch pad and delayed launches. This was one of the largest artificial non-nuclear explosions in human history and was visible that evening 22 miles (35 kilometres) away at Leninsk (See Tyuratam).[45]

Soon after lift-off, due to unexpected eddies and counter-currents at the base of Block A (the first stage), the N-1 experienced an uncontrolled roll beyond the capability of the control system to compensate. The KORD computer sensed an abnormal situation and sent a shutdown command to the first stage, but as noted above, the guidance program had since been modified to prevent this from happening until 50 seconds into launch. The roll, which had initially been 6° per second, began rapidly accelerating. At T+39 seconds, the booster was rolling at nearly 40° per second, causing the inertial guidance system to go into gimbal lock and at T+48 seconds, the vehicle disintegrated from structural loads. The interstage truss between the second and third stages twisted apart and the latter separated from the stack and at T+50 seconds, the cutoff command to the first stage was unblocked and the engines immediately shut down. The upper stages impacted about 4 miles (7 kilometers) from the launch complex. Despite the engine shutoff, the first and second stages still had enough momentum to travel for some distance before falling to earth about 9 miles (15 kilometers) from the launch complex and blasting a 15-meter-deep (50-foot) crater in the steppe.[46] This N1 had dummy upper stages without the rescue system. The next, last vehicle would have a much more powerful stabilization system with dedicated engines (in the previous versions stabilization was done by directing exhaust from the main engines). The engine control system would also be reworked, increasing the number of sensors from 700 to 13,000.[37][47]

The start and lift-off went well. At T+90 seconds, a programmed shutdown of the core propulsion system (the six center engines) was performed to reduce structural stress on the booster. Because of excessive dynamic loads caused by a hydraulic shock wave when the six engines were shut down abruptly, lines for feeding fuel and oxidizer to the core propulsion system burst and a fire started in the boattail of the booster; in addition, the #4 engine exploded. The first stage broke up starting at T+107 seconds and all telemetry data ceased at T+110 seconds. The launch escape system activated and pulled the Soyuz 7K-LOK to safety. The upper stages were ejected from the stack and crashed into the steppe. An investigation revealed that the abrupt shutdown of the engines led to fluctuations in the fluid columns of the feeder pipes which ruptured and spilled fuel and oxidizer onto the shut down, but still hot, engines. A failure of the #4 engine turbopump was also suspected. It was believed that the launch could have been salvaged had ground controllers sent a manual command to jettison the first stage and begin second stage burn early.[48]

Vehicle serial number 8L was prepared for August 1974. It included a regular 7K-LOK Soyuz 7K-LOK and a regular LK module-spacecraft of the L3 lunar expedition complex. It was intended for a Moon flyby and uncrewed landing in preparation for a future crewed mission. As the N1-L3 program was canceled in May 1974, this launch never took place.

There is confusion among Russian online sources as to whether N1-L3 (Russian: Н1-Л3) or N1-LZ (Russian: Н1-ЛЗ) was intended, because of the similarity of the Cyrillic letter Ze for "Z" and the numeral "3". Sometimes both forms are used within the same Russian website (or even the same article).[34] English sources refer only to N1-L3. The correct designation is L3, representing one of the five branches of Soviet lunar exploration. Stage 1 (Л1) would be a crewed circumlunar flight (only partially realized); stage 2 (Л2) would be an unmanned lunar rover (realized as Lunokhod), stage 3 (Л3) would be the crewed landing, stage 4 (Л4) would be a manned spacecraft in lunar orbit, and stage 5 (Л5) would be a heavy manned lunar rover to support a crew of 3–5 people.[49][50]